1. Field of the Invention
This invention relates to a process for preparing a cathode structure for impregnated cathodes used in electric discharge tubes.
2. Description of the Prior Art
Cathode is an essential constituent of electric discharge tubes, upon which efficiency and life of the electric discharge tubes are decided. Characteristics required for the cathodes for electric discharge tubes are as follows:
(a) high electron emission efficiency;
(b) high current density;
(c) uniform emission energy;
(d) stable operation;
(e) long life;
(f) sufficient resistance to the vacuum pressure imparted to the electric discharge tube; and
(g) no electron emission at any part other than the predetermined electron emitting surface.
Impregnated cathodes can be given as those having the above-described characteristics.
An impregnated cathode is prepared by impregnating a porous high-melting-point metal base such as of porous tungsten with an electron emitting material comprising a compound oxide based on barium. The thus prepared impregnated cathode is usually mounted on a heater sleeve with a heater accommodated therein. During operation of the cathode, the compound oxide impregnated in the porous metal base is heated by the heater and reduced at the activation temperature into free metals which diffuse throughout the surface of the porous metal base and form a single atomic layer. The thus formed single atomic layer is designed to have greatly reduced work function compared with the tungsten, enabling efficient electron emission.
The cathode structure for such impregnated cathodes has conventionally been prepared in the following manner:
(a) a high-melting-point metal powder is shaped by powder extrusion method and then sintered;
(b) the thus sintered porous high-melting-point metal is infiltrated with an acrylic resin as a lubricant so as to facilitate machining thereof;
(c) upon completion of machining into predetermined dimensions of cathode base, the infiltrated acrylic resin is removed; and
(d) the cathode base is soldered or welded onto a heater sleeve.
As described above, fabrication of a cathode structure for impregnated cathodes requires such considerable time, equipments and materials.
According to the above prior art method, an impregnated cathode can be obtained through a long process (molding and sintering.fwdarw.acrylic resin infiltration.fwdarw.machining.fwdarw.acrylic resin removal.fwdarw.bonding), leading to increase in the manufacturing cost and facility cost, disadvantageously. Meanwhile, shaping of the porous high-melting-point metal base includes a molding step and a sintering step, increasing the working time, and besides deformation which occurs during sintering makes it difficult to obtain a desired size of product.